Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer for which an electrophotography method or an electrostatic recording method is employed.
Description of the Related Art
In an electrophotography type image forming apparatus, when a difference in a conveyance speed of a recording material occurs between a transfer nip portion and a fixing nip portion, defective images can be generated in some cases.
That is, since a temperature of a fixing roller or a pressure roller of the fixing nip portion fluctuates due to a number of passing recording materials or a number of passage times, a thermal expansion amount changes depending on a situation and appears as a change in an outer diameter. Thus, the conveyance speed of the recording material at the fixing nip portion changes, and a difference from the conveyance speed of the recording material occurs at the transfer nip portion.
If the conveyance speed of the fixing nip portion is faster than the conveyance speed of the transfer nip portion, the recording material is pulled by the fixing nip portion between the fixing nip portion and the transfer nip portion, and there is a concern that a disturbance can occur in an image when an unfixed image on an image bearing member is to be transferred to the recording material at the transfer nip portion.
On the other hand, if the conveyance speed of the recording material by the fixing nip portion is slower than the conveyance speed of the recording material by the transfer nip portion, the recording material begins to sag between the fixing nip portion and the transfer nip portion. The state where the recording material sags as above will be described below as “the recording material forms a loop”. When the recording material forms a loop as described above, if the loop becomes too large, the recording material might be strongly pressed on and rubbed by a conveyance guide for guiding conveyance of the recording material between the transfer nip portion and the fixing nip portion. Moreover, if the loop is excessively formed, the recording material waves between the transfer nip portion and the fixing nip portion and can be rubbed by a ceiling of a conveyance path. As a result, the unfixed image transferred on the recording material is disturbed, and a defective image is generated.
Thus, the following proposal as below is made that the defective image is suppressed by causing a predetermined loop to be formed between the transfer nip portion and the fixing nip portion and by preventing the recording material from being pulled or sagging by that.
That is, a loop detection sensor for detecting a loop amount of the recording material and a loop detection flag for turning ON/OFF the loop detection sensor are provided on a conveyance guide between the fixing nip portion and the transfer nip portion. Then, according to a detection result of the loop detection sensor, the conveyance speed of the fixing nip portion is switched to a first speed slower than the conveyance speed of the transfer nip portion and a second speed faster than the conveyance speed of the transfer nip portion, and the loop amount is maintained within a predetermined range. By means of this loop control, while the recording material is sandwiched/conveyed by both the transfer nip portion and the fixing nip portion, image can be formed while the loop amount is held substantially constant (Japanese Patent Application Laid-Open No. H05-107966 and Japanese Patent Application Laid-Open No. H07-234604).
However, since the loop amount between the transfer nip portion and the fixing nip portion is substantially constant at all times, if a size of this loop is set large, for example, a recording material trailing edge might bounce by stiffness of the recording material in some cases when a recording material railing edge exits out the transfer nip portion. By means of this bouncing of the recording material, there is a concern that an image surface of the recording material might be rubbed by a conveyance path ceiling or the like, and a defective image is incurred.
In order to solve this problem of bouncing of the recording material trailing edge, a technology is proposed that the size of the loop of the recording material is set constant and image formation is performed, and the conveyance speed of the fixing nip portion is switched so that the loop of the recording material is decreased at predetermined timing before the recording material trailing edge passes through the transfer nip portion. As a result, the recording material trailing edge passes through the transfer nip portion in a state where the loop of the recording material is small, and bouncing of the recording material trailing edge is prevented (Japanese Patent No. 4136392).
However, various media need to be handled in response to market needs, and handling of image formation on thicker cardboards is also in demand. On the other hand, size reduction of an image forming apparatus is in demand, and new problems occur in a course of handling of these problems.
FIGS. 8A and 8B illustrate explanatory views of an attitude of a recording material P between a transfer nip portion N1 and a fixing nip portion N2. FIGS. 8A and 8B are for describing the problem of the present invention and do not illustrate conventional examples. Therefore, the same reference numerals are given to the same constituent portions as those of the embodiment of the present invention. In promoting size reduction of an apparatus, the transfer nip portion N1 and the fixing nip portion N2 needs to be brought closer to each other. By bringing the distance between the transfer nip portion N1 and the fixing nip portion N2 closer to each other, when the recording material P conveyed from the transfer nip portion N1 is curled or the like, a recording material leading edge cannot be easily introduced into the fixing nip portion N2 stably and smoothly.
In order to stably introduce even the curled recording material P into the fixing nip portion, an angle A formed by a nip tangential line Lt of the transfer nip portion N1 and a nip tangential line Lf of the fixing nip portion N2 needs to be designed larger than the conventional. By constituting as above, the recording material leading edge can easily follow a conveyance guide 73, and the recording material leading edge can be stably introduced into the fixing nip portion N2. In such an apparatus, when a cardboard is to be fed, at a moment when the recording material trailing edge Pe exits out the transfer nip portion N1, the recording material trailing edge Pe cancels the loop vigorously toward the nip tangential line Lf by rigidity (stiffness) of the recording material P (see FIG. 8A). As a result, the recording material trailing edge Pe is slapped onto the conveyance guide 73, an unfixed image in the vicinity of the recording material trailing edge is disturbed, and a defective image might occur in some cases (see FIG. 8B).
Therefore, since the larger a thickness of the recording material P is, the higher the rigidity becomes, a force to cancel the loop of the recording material gets stronger, and the defective image can occur more easily. Moreover, the closer the distance between the transfer nip portion N1 and the fixing nip portion N2 is, and the larger the angle A formed by the nip tangential line Lt of the transfer nip portion N1 and the nip tangential line Lf of the fixing nip portion N2 is, the force to cancel the loop of the recording material gets strong, and the defective image can occur more easily, which is a problem.
Moreover, as in Japanese Patent No. 4136392, even if control for reducing the loop of the recording material is executed before the recording material trailing edge Pe exits out the transfer nip portion N1, in the case of the cardboard, the loop is to be canceled by rigidity of the recording material toward a tangential line direction of the nip tangential line Lf of the fixing nip portion N2. Therefore, the recording material trailing edge Pe is slapped onto the conveyance guide 73, and occurrence of a defective image cannot be suppressed.